1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device, which enhances an aperture ratio and minimizes off current, and a method for fabricating the same.
2. Discussion of the Related Art
As an information-oriented age has been introduced, requirements for display devices have been variously increased, and in order to satisfy this increase, various flat display devices, such as a liquid crystal display (LCD), a plasma display panel (PDP), an electro luminescent display (ELD), a vacuum fluorescent display (VFD), etc., have been researched, and some flat display devices have been already applied to several equipment.
Among the above flat display devices, a liquid crystal display, which replaces a conventional cathode ray tube (CRT), is most widely used as a mobile image display device due to its advantages, such as light weight, thin profile, and low power consumption, and is variously developed to be used as a television receiving a broadcast signal and displaying an image and a monitor of a computer in addition to the mobile image display device, such as a monitor of a notebook computer.
In general, a liquid crystal display device includes a thin film transistor substrate, a color filter substrate, and a liquid crystal layer formed between the two substrates.
The thin film transistor substrate includes a plurality of gate lines and a plurality of data lines, which are disposed lengthwise and crosswise to define a plurality of pixel regions, thin film transistors respectively formed at the crossings of the gate lines and the data lines and serving as switching elements, and pixel electrodes respectively formed on the pixel regions.
The color filter substrate includes color filters to express colors, and a black matrix to divide the color filters from each other and prevent the reflection of external light.
The thin film transistor substrate of the liquid crystal display device requires a semiconductor process and a plurality of mask processes and has a complicated fabricating method, and is thus a main factor raising the fabricating cost of the liquid crystal display device.
Therefore, a method, in which a semiconductor layer and a source/drain pattern including data lines and source/drain electrodes are formed by one mask process to reduce the number of mask processes required, has been proposed. In this method, the source/drain pattern is formed by a first etching process to form the same pattern as the semiconductor layer and a second etching process to separate the source/drain electrodes.
Here, since the semiconductor layer is formed through one time of the etching process but the data lines on the semiconductor layer are formed through two times of the etching process, the line width of the data lines is smaller than the line width of the semiconductor layer disposed under the data lines.
Therefore, the semiconductor layer has a structure, which is protruded from both sides of the data lines to the pixel regions. The pixel electrodes formed in the pixel regions must be separated from the data lines and the semiconductor layer so as to reduce the effect of parasitic capacitance.
Thereby, the area of the pixel electrodes is reduced as large as the area of the protrusions of the semiconductor layers from the data lines.
Further, the semiconductor layer formed under the data lines is made of amorphous silicon containing hydrogen, and absorbs light when light from a backlight unit is incident upon the semiconductor layer. Due to the absorbance of light by the semiconductor layer, off current is increased even when the thin film transistors are turned off.
More specifically, since bonding between silicon and hydrogen in the semiconductor layer made of amorphous silicon is exceedingly weak, when light is irradiated on the semiconductor layer, the bonding between silicon and hydrogen is broken off and thus electrons move to the surface of the semiconductor layer (i.e., photo current).
The above photo current causes the lowering of operating characteristics of the thin film transistors.
Further, as the off current is increased, an on/off current ratio (Ion/Ioff) affecting characteristics of the liquid crystal display device is decreased, and thus lowers the productivity and reliability of a product due to shortening of the life span of the liquid crystal display device and deepening of an after-image.